Firstly, the over/under goals markets are popular betting markets that cover the total number of goals in a match (both teams). Over/under 0.5 goals is the lowest number you can bet on in these traditional markets, and the bets typically increase by one goal such as over/under 1.5, 2.5, 3.5, 4.5, etc.
In the over/under goals markets, there is no possibility for a draw or refund, which is what the 0.5 decimal means. As a team cannot score half a goal, there is no possibility for a draw or refund on these bets, you either win or lose. One of the most popular goals markets for betting is the over/under 2.5 goals markets, as it is often priced around evens.
Over 0.5 goals in a match mean that 1 or more goals are required to win the bet, and 0 goals mean the bet loses. The over 0.5 goals market applies to 90-minute matches (plus additional time) but does not include extra time.
The same applies to under 0.5 goals, 1 or more goals and you lose, but 0 goals mean you win. Also, the under 0.5 goals betting market covers 90-minute games plus additional time but does not include extra time.